1. Field of the Invention
Apparatuses consistent with the present invention relate to a voltage controlled oscillator (VCO), and more particularly, to a complementary metal oxide semiconductor (CMOS) VCO applying a gate bias voltage independent of a power supply voltage VDD using a current mirror instead of a tail current source, and a stable power instead of the noisy power supply voltage VDD to an inverter buffer using the current mirror so as to reduce a phase noise.
2. Description of the Related Art
In general, VCOs are integrated circuit (IC) chips varying voltages to oscillate super high frequency having desired frequencies. Such a VCO is mainly used in analog synthesizers, mobile communication terminals, personal mobile communications terminals, base stations, and other communications equipment known in the art, and outputs a desired oscillator frequency using a voltage applied from an external source.
In the VCO, an output frequency linearly varies with a linear variation in a voltage. Thus, a frequency of an output signal is proportional to an intensity of an input signal. Therefore, the VCO can be effectively used in a frequency modulator.
FIG. 1 is a circuit diagram of a conventional VCO including a noise filter. Referring to FIG. 1, the conventional VCO includes an inductance-capacitance (LC) tank 110 including inductors L11 and L12 and varactor diodes D11 and D12 to oscillate a frequency, a negative resistor 120 sustaining the oscillation of the LC tank 110, a tail current source 130 allowing a current to symmetrically flow in the VCO, and a noise filter 140 preventing noise generated from the tail current source 130 from flowing in the VCO.
The LC tank 110 includes first and second resonators symmetric with respect to a control voltage Vc, the first resonator includes a first inductor L11 and a first varactor diode D11 connected to each other, and the second resonator includes a second inductor L12 and a second varactor diode D12 connected to each other.
The negative resistor 120 includes first and second N-channel Metal Oxide Semiconductor Field Effect Transistors (NMOS FETs) MN11 and MN12, and a drain of the first NMOS FET MN11 is connected to a node P to which an end of the first inductor L11 and anode of the first varactor diode D11 are connected in common. A gate of the first NMOS FET MN11 is connected to a node N to which an end of the second inductor L12 and an anode of the second varactor diode D12 are connected in common. Symmetrically, a drain of the second NMOS FET MN12 is connected to the node N, and a gate of the second NMOS FET MN12 is connected to the node P.
The tail current source 130 includes an NMOS FET.
The noise filter 140 is connected between sources of the first and second NMOS FETs MN11 and MN12 and a drain of the tail current source 130, and includes an inductor L13 and a bypass capacitor C13.
The conventional VCO shown in FIG. 1 is supplied with a power supply voltage VDD from a power source and varies capacitances of the first and second varactor diodes D11 and D12 using the control voltage Vc to control an oscillator frequency output from the LC tank 110. The negative resistor 120 having a negative resistance is added to the LC tank 110 so as to sustain the oscillator signal.
The conventional VCO shown in FIG. 1 is a differential VCO and outputs an oscillator signal having a phase difference of 180° via the nodes P and N as output nodes. Capacitors C11 and C12 connected to the nodes P and N, respectively, are bypass capacitors bypassing the output oscillator signal. A drain of the NMOS FET of the tail current source 130 is connected to a source of the negative resistor 120 so that the same current flows in sources of the first and second NMOS FETs MN11 and MN12 of the negative resistor 120.
However, a 1/f noise generated from the NMOS FET of the tail current source 130 and upconverted appears in an output of the conventional VCO. The 1/f noise is called a flicker noise. The inductor L13 having a large inductance is inserted between the negative resistor 120 and the tail current source 130 to control the 1/f noise. A noise that has not passed through the inductor L13 is bypassed by the bypass capacitor C13.
Thus, the conventional VCO shown in FIG. 1 must use an additional passive element to control the 1/f noise of the tail current source 130.
FIG. 2 is a circuit diagram of inverter buffers connected to output nodes of a conventional VCO. Since the conventional VCO shown in FIG. 1 has a differential structure, an output node is divided into nodes P and N. Thus, input nodes of the inverter buffers are each divided into nodes P and N.
A first inverter 210 shown in FIG. 2 includes a PMOS FET MP21 and an NMOS FET MN21 that have different polarities, and their gates and drains are coupled together. A power supply voltage VDD is connected to a source of the PMOS FET MP21, and a source of the NMOS FET MN21 is connected to a ground voltage VSS. Components of a second inverter buffer 220 are connected using the same method by which the components of the first inverter 210 have been connected. Components of third and fourth inverter buffers 230 and 240 are connected using the same method by which the components of the first inverter buffer 210 have been connected, and thus their descriptions will be omitted.
The first inverter buffer 210, connected to an output node VIP of the conventional VCO shown in FIG. 1, is driven by the power supply voltage VDD and increases an amplitude of an output oscillator signal of the conventional VCO. Thus, a full swing is output to increase a power of the output oscillator signal. As a result, a phase noise of the conventional VCO can be controlled using an inversely proportional relationship of the power to the phase noise.
However, the phase noise of the conventional VCO is increased by an effect of a noise of the power supply voltage VDD connected to the first inverter buffer 210. In other words, the characteristic of the phase noise is deteriorated. The second, third, and fourth inverter buffers 220, 230, and 240 operate in the same way, and thus the phase noise is increased.
Accordingly, in a scheme of controlling a phase noise of a conventional VCO, an additional passive element is used to control a 1/f noise generated from a tail current source and upcoverted. Also, inverter buffers are used to increase a power so as to control the phase noise of an oscillator signal of the conventional VCO. As a result, the phase noise is increased by effects of noises of power supply voltages VDD of the inverter buffers.